Re: [PATCH v2 11/11] sched: replace capacity by activity

[Vincent Guittot]

On 2 June 2014 08:21, Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx> wrote:
> On 05/29/2014 07:25 PM, Peter Zijlstra wrote:
>> On Fri, May 23, 2014 at 05:53:05PM +0200, Vincent Guittot wrote:
>>> The scheduler tries to compute how many tasks a group of CPUs can handle by
>>> assuming that a task's load is SCHED_LOAD_SCALE and a CPU capacity is
>>> SCHED_POWER_SCALE.
>>> We can now have a better idea of the utilization of a group fo CPUs thanks to
>>> group_actitvity and deduct how many capacity is still available.
>>>
>>> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>>> ---
>>
>> Right, so as Preeti already mentioned, this wrecks SMT. It also seems to
>> loose the aggressive spread, where we want to run 1 task on each 'core'
>> before we start 'balancing'.
>
> True. I just profiled the ebizzy runs and found that ebizzy threads were
> being packed onto a single core which is SMT-8 capable before spreading.
> This was a 6 core, SMT-8 machine. So for instance if I run 8 threads of
> ebizzy. the load balancing as record by perf sched record showed that
> two cores were packed upto 3 ebizzy threads and one core ran two ebizzy
> threads while the rest of the 3 cores were idle.
>
> I am unable to understand which part of this patch is aiding packing to
> a core.  There is this check in this patch right?
>
> if (sgs->group_capacity < 0)
>                 return true;
>
> which should ideally prevent such packing? Because irrespective of the
> number of SMT threads, the capacity of a core is unchanged. And in the
> above scenario, we have 6 tasks on 3 cores. So shouldn't the above check
> have caught it?

yes, it should. the group_capacity should become < 0 because the CPU
are fully loaded and the activity reach the max capacity value +
nr_running

>
> Regards
> Preeti U Murthy
>>
>> So I think we should be able to fix this by setting PREFER_SIBLING on
>> the SMT domain, that way we'll get single tasks running on each SMT
>> domain before filling them up until capacity.
>>
>> Now, its been a while since I looked at PREFER_SIBLING, and I've not yet
>> looked at what your patch does to it, but it seems to me that that is
>> the first direction we should look for an answer to this.
>>
>
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